A high-temperature gas reactor has a reactor core, into which fuels for the high-temperature gas reactor are introduced, which reactor core is made of graphite that has a large thermal capacity and keeps its crystalline structure in good condition at high temperatures. The high temperature gas reactor employs, as coolant gas, a gas such as helium gas, which is assessed as being safe because helium gas does not react even at high temperatures. The employment of helium gas makes it possible to take the coolant gas away safely even when the temperature around the outlet of the coolant gas is high. Therefore, the coolant gas, which has been heated up to a high temperature of about 1000° C., is used as a heat source in a wide variety of fields such as hydrogen production and chemical plants, as well as power plants.
Fuels for the high-temperature gas reactor typically comprises a fuel kernel and a coating layer with which the fuel kernel is covered. The fuel kernel is a small particle with a diameter of about 350 to 650 μm, made by sintering uranium dioxide into a physical state like ceramics.
The coating layer generally comprises concentrically laminated sub-coating layers. When the coating layer has four sub-coating layers, they are called “the first sub-coating layer”, “the second sub-coating layer”, “the third sub-coating layer”, and “the fourth sub-coating layer” from the sub-coating layer adjacent to the fuel kernel. The diameter of the particle comprising the fuel kernel and four sub-coating layers is typically about 500 to 1000 μm.
The fuel kernels may be produced in the following way with an apparatus for producing ammonium diuranate particles. Firstly, a uranium oxide in the form of powder is dissolved in nitric acid, which produces a uranyl nitrate solution. Then, the uranyl nitrate solution is mixed with pure water, a thickening agent, and other chemicals, if necessary, and the mixture is stirred. A feedstock liquid to be dripped is obtained by this process. The feedstock liquid is stored in a feedstock liquid reservoir. The feedstock liquid thus prepared is cooled to a predetermined temperature, the viscosity thereof is adjusted, and then it is transferred to a dripping nozzle device. The dripping nozzle device has one nozzle with a small diameter. The transferred feedstock liquid falls in drops from the end of the nozzle into an aqueous solution of ammonia. The uranyl nitrate included in the drops, which have fallen into the aqueous solution of ammonia, changes into ammonium diuranate from the surfaces of the drops through the reaction. If the drops including uranyl nitrate reside in the solution for a time period enough to complete the reaction, uranyl nitrate in the central part of each drop is changed to ammonium diuranate.
The drops dripped from the nozzle pass through an atmosphere of ammonia gas in the process of falling toward the surface of the aqueous ammonia solution. This ammonia gas brings about gelation on the surface of each drop, which forms a film there. The drops with the film are protected from deformation to some extent, caused by the impact that occurs when the drops fall to and hit the surface of the aqueous ammonia solution. If uranyl nitrate included in the drops that have fallen into the solution reacts with ammonia sufficiently, ammonium diuranate particles, which may sometimes be abbreviated to “ADU particles”, are formed.
The ADU particles thus formed are washed, dried, and then calcined in the atmosphere, which changes the ADU particles in to uranium trioxide particles. The obtained uranium trioxide particles are reduced and sintered, through which steps the uranium trioxide particles are changed into uranium dioxide particles with high density, in a condition like ceramics. The uranium dioxide particles are sieved, or classified, and fuel kernel particles with a diameter within a predetermined range are obtained.
After the coating layer is formed on the kernel particles, the fuels for the high-temperature gas reactor are fabricated into fuel compacts or fuel pebbles. The fuel compacts or pebbles are obtained by pressing or molding the fuels with a graphite matrix material made of graphite powder, a binder and other components into cylinders with contents, hollow cylinders, or spheres, and calcining the pressed or molded. See “Genshiro Zairyo Handbook”, or “A Handbook about Nuclear Reactor Materials”, published by The Nikkan Kogyo Shimbun, Ltd. on Oct. 31, 1977, and “Genshiryoku Handbook”, or “Nuclear Energy Handbook”, published by Ohmsha, Ltd. on Dec. 20, 1995.
In a process of producing fuels for the high-temperature gas reactor described in “Genshiro Zairyo Handbook”, a feedstock liquid, from which ammonium diuranate particles are prepared, is obtained by adding pure water and a thickening agent to a uranyl nitrate stock solution and stirring the obtained mixture. However, the handbook lacks an explanation of detailed conditions necessary for the preparation. A person skilled in the art of the field of nuclear energy, reading only this book, is unable to produce ammonium diuranate particles with good sphericity and a flawless inside structure.
Uranyl nitrate was prepared by reacting nitric acid with a uranium oxide, for example, triuranium octaoxide, in accordance with the following reaction formula:U3O8+8HNO3→3UO2(NO3)2+2NO2↑+4H2O  (1)
Based on formula (1), the skilled artisan thought that when 2.66 moles or more of nitric acid was used for 1 mole of uranium, uranyl nitrate could stoichiometrically be prepared without leaving unreacted uranium oxide, for example, triuranium octaoxide. However, the conventional method that used an excess amount of nitric acid cost much and the nitrogen content in the waste fluid was inevitably raised, which resulted in an increased burden on the environment. On the other hand, when less than 2.66 moles of nitric acid was used for 1 mole of uranium, stoichiometrically, unreacted uranium oxide, for example, unreacted triuranium octaoxide was left after the reaction. The unreacted uranium oxide was sometimes included in a uranyl nitrate stock solution, which led to a failure in producing ammonium diuranate particles as previously planned. Besides, the skilled artisan could not expect that ammonium diuranate particles with good sphericity would be produced.